In recent times, much research effort has been done on solar heat collecting systems. This is primarily so because of the existing fact of a shortage of fossil fuels. Typical solar heat collecting systems comprise a solar collecting panel having an exteriorally exposed sunlight transmitting panel and at least one interiorally disposed sunlight transmitting panel, a solar collecting plate spaced apart from and behind the sunlight transmitting panels and a heat absorption fluid passing behind the collector plate to provide heat exchange with the collector plate itself. In actual operation, the sunlight passes through the sunlight transmitting panels and hits the collector plate. The collector plate converts the solar energy into heat energy; and, the heat absorption fluid which usually passes over one of the surfaces of the collector plate, provides heat exchange with the thermally warmed collector plate. Thereafter, the now heated absorption fluid is typically conveyed away to a remote place within the building structure for storage until it is subsequently utilized to warm the interior of the building.
The collector plates, utilized to convert solar energy to heat energy, are well known and are usually comprised of any suitable metal or like material of high solar absorptivity and are typically painted a dark highly absorptive color such as flat black. Metal from which satisfactory collector plates can be made are copper, aluminum, steel and galvanized iron. Aluminum is perhaps the most commonly used collector.
Heat absorption fluids which pass over the collector plate to provide heat exchange therewith are also known. Typical examples of heat absorption fluids utilized in solar heating systems are water, air, ethylene glycol, propylene glycol and other heat exchange fluids.
The heat produced during the light of the day must be stored so that it can be utilized to provide heating during the night and at times when the sun is not shining. This invention is concerned with an efficient heat storage system.
Many heat storage materials utilized to store heat energy for solar heating systems are phase change materials. That is, the heat storage material undergoes phase changes, including changes from solid to liquid form and changes from one crystalline form to another, during heat exchange as the heat storage material gains and loses heat. A typical example of a commonly utilized heat storage material which is a phase change material is Glauber's salt.
Normally one would not be concerned with whether or not a heat storage material undergoes phase changes during heat exchange. However, it has been found that many of the more efficient heat storage materials such as Glauber's salt, which have extremely desirable heat storage characteristics, also possess some unique problems. They are incongruently melting materials. During the fluctuations in temperature involved in the heat exchange between the storage material and the heat absorption fluid, materials such as Glauber's salt undergo stratification. As a result, they form layers of crusted material. This stratification may reduce the heat storage capacity of the material such as Glauber's salt, by more than 50% in a relatively few cycles. Thus, many heat storage materials having extremely desirable heat storage characteristics cannot be utilized successfully because of their inherent stratification problems.
Sodium sulfate decahydrate, commonly known as Glauber's salt, is one example among a class of incongruently melting salts generally referred to as eutectic salts. Others include those listed in the table below:
TABLE I ______________________________________ EUTECTIC SALTS Storage Melting Density Heat of Fusion Medium Temp .degree.F. lb/ft.sup.3 Btu/lb Btu/ft.sup.3 ______________________________________ Na.sub.3 PO.sub.4 . 12H.sub.2 O 150 89 82 7,300 NaOH . H.sub.2 O 148 105 117 12,200 NaC.sub.2 H.sub.3 O.sub.2 . 3H.sub.2 O 136 81 114 9,200 NA.sub.2 S.sub.2 O.sub.3 . 5H.sub.2 O 119 103 90 9,300 Ca(NO.sub.3).sub.2 . 4H.sub.2 O 117 116 66 7,650 FeCl.sub.3 . 6H.sub.2 O 97 101 96 9,700 Na.sub.2 CO.sub.3 . 12H.sub. 2 O 97 95 114 10,800 Na.sub.2 CO.sub.3 . 10H.sub.2 O 93 90 108 9,750 Na.sub.2 SO.sub.4 10H.sub.2 O 89 91 108 9,850 ______________________________________
As previously mentioned during the phase change from solid to liquid, these eutectic salts, and particularly Glauber's salt, are highly useful since the phase change involves a tremendous amount of heat which is either absorbed or rejected during phase change. This heat storage capacity is called "latent heat".
Again, as heretofore mentioned, incongruently melting eutectic salts exhibit a detrimental characteristic called stratification which results in a loss of latent heat storage capacity. For example, with sodium sulfate decahydrate, Glauber's salt, at 90.3.degree. F. the solid crystals melt and change to a saturated solution of 85% sodium sulfate in water and 15% anhydrous sodium sulfate. The anhydrous salt is more dense than the solution; consequently the white granular anhydrous salt settles to the bottom of the container. Increasing the temperature of the solution does not increase solution solubility.
Upon continued cycling, three distinct layers can be identified within the container. The bottom layer will be anhydrous sodium sulfate unable to mix with the water, the next layer will be the decahydrate crystals and the top layer will be free water unable to mix with the bottom anhydrous layer.
Other heat storage materials which involve heat storage capacity because of latent heat of fusion during phase change include organic paraffin materials. Such paraffins are congruently melting compounds which do not exhibit stratification and super cooling properties. They do, however, self insulate upon crystallization and are flammable. When liquid paraffins begin to solidify they to do so first at the edges which are exposed to contact with the container in which they rest. Crystalline structure of the solid paraffins at or near the outer edges of the mass of material is such that it effectively insulates the liquid paraffin. In other words, it reduces the heat flow rate from the liquid paraffin to the walls of the container through which heat is transferred for use to condition space. This characteristic reduces the effectiveness and usefulness of the heat stored in the center of a paraffin mass. It has now been found that this problem can be solved by molding a heat conductive transfer material, such as steel wool directly within the paraffin mass of material. The metal fibers are excellent heat conductors which conduct heat effectively both into and out of the center mass of paraffin. Thus their inherent transfer limitations are overcome.
Accordingly, one object of this invention is to provide a heat transfer article which can be used with congruently melting compounds such as paraffins in order to make the heat transfer far more effective for such paraffins.
Yet another object of this invention is to provide a heat storage material which can be effectively used with incongruently melting heat storage materials, such as eutectic salts like Glauber's Salt, which will effectively prevent stratification and decreasing efficiency of the heat storage material.
Yet another object of this invention is to provide a heat storage container made from essentially commonly available building concrete which has its heat tranfer characteristics augmented significantly by molding directly into the concrete material a certain portion of an effective heat transfer metal in the form of filings, flakes, chips or the like.
Yet another object of this invention is to provide a heat storage container which is essentially a concrete block structure which can be used for passive solar heating via a solar heat storage system or for direct solar radiation by exposure directly to solar radiation for absorption directly into the material. The concrete block building structure of this invention may be used to form wall structures if desired. They may be used for flooring, or any other conventional use for which building blocks may be used.